Merge pull request #46 from leaflet-extras/update-leaflet

Bump version of Leaflet, some comment typos
Replace attribution placeholders recursively
Merge pull request #48 from leaflet-extras/fix-attribution

Replace attribution placeholders recursively
Merge branch 'master' of github.com:leaflet-extras/leaflet-providers into preview-page
Display baselayer + all overlay(s) currently in view